176 (i.e.180) males and females 30 - 60 years-of-age referred to specialist clinic in Bø in Telemark. No interactive diseases with periodontal disease and willing to participate by signing an informed consent. Patients should have >4 pockets 5 - 8mm deep, bacterial diagnosis which indicates the use of Metronidazole only (to prohibit differences due to antibiotic choice). Included patients should not have known allergy or ill-reaction to this drug. Microbiological diagnosis will be performed by PCR and culture and a second microbiological "shadow-test" will be performed at a parallel microbiological service. The primary microbiological diagnosis will be based on both culture and PCR on samples obtained by curettes.